Field of the Invention
The invention relates to an organic light emitting diode and a display device including the same that improve a display quality by minimizing color inversion and color shift according to viewing angles.
Discussion of the Related Art
As one of display devices, an organic light emitting diode (OLED) display device, which may be referred to as an organic electroluminescent display device, has high brightness and low driving voltage. In addition, because it is self-luminous, the OLED display device has an excellent contrast ratio and an ultra thin thickness, and has a response time of several micro seconds, and thus there are advantages in displaying moving images without delays. The OLED display device is stable under low temperatures and since the OLED display device is driven by low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits. Accordingly, the OLED display device is widely used for various information technology (IT) devices such as a television, a monitor, a mobile phone, and so on.
Hereinafter, a structure of an OLED display device will be described in more detail.
FIG. 1 is a cross-sectional view of schematically illustrating a pixel region of an organic light emitting diode (OLED) display device according to the related art.
In FIG. 1, the OLED display device 1 according to the related art includes a first substrate 10 having an array element and organic light emitting diodes E and a second substrate 70 for encapsulation opposite to the first substrate 10.
The array element on the first substrate 10 includes switching thin film transistors (not shown) connected to gate and data lines (not shown) and driving thin film transistors DTr connected to the organic light emitting diodes E. Each of the organic light emitting diodes E includes a first electrode 47 connected to the driving thin film transistor DTr, an organic light emitting layer 55 and a second electrode 58.
The organic light emitting layer 55 includes an organic light emitting material that emits red, green and blue in respective sub pixels SP1, SP2 and SP3.
Light emitted from the organic light emitting layer 55 is outputted through the first electrode 47 or the second electrode 58, and thus the OELD display device 1 displays an image.
Meanwhile, in the OLED display device 1 having the above-mentioned structure, a microcavity effect is used to improve luminous efficiency and color purity of a displayed image.
The microcavity effect is a phenomenon where the light emitted from the organic light emitting layer 55 is repeatedly and selectively reflected between specific layers and is transmitted with changed spectrum and increased optical intensity through the first electrode 47 or the second electrode 58 to thereby improve the color purity and brightness of light finally outputted.
However, the brightness of light emitted by the OLED display device using the microcavity effect decreases as a viewing angle increases, and the main peak wavelength of light shifts from a long wavelength to a short wavelength to cause a color shift. Here, the viewing angle is defined as zero degree when a user views the image at the front and increases as a viewing point of the user goes to a side with respect to the front. The viewing angle is within a range of 0 to 90 degrees up and down and left and right with respect to the front.
FIG. 2 is a view illustrating color shifts according to viewing angles in a pixel including red, green and blue sub pixels of the related art OLED display device when a white image is displayed.
In FIG. 2, when the user views the image of the OLED display device 1 at the viewing angle of zero, normal white is displayed and viewed. On the other hand, when the user views the image at the viewing angles of 45 degrees and 60 degrees, color coordinates are shifted, and white different from the normal white is displayed and viewed. Namely, as the viewing angle increases, the brightness of light emitted by the OLED display device 1 is decreased, and the main peak wavelength of the light emitted by the OLED display device 1 is shifted. Thus, the color of an image viewed at a predetermined angle, for example, 45 degrees or 60 degrees as shown in FIG. 2, is different from the color of the image viewed at the front (e.g., at 0 degrees), which is problematic.
FIG. 3 is a graph illustrating changes of brightness according to viewing angles of 0 to 80 degrees in the related art OLED display device.
In FIG. 3, when an image is viewed at the front and the viewing angle is 0, the brightness is 100%. As the viewing angle is changed from 0 degree to 60 degrees, the brightness of red, green, blue, and white light is gradually lowered.
For example, while the white light has the brightness of 100% at 0 degree, the white light has the brightness of about 24% at the viewing angle of 60 degrees. In addition, while the red, green and blue light has the brightness of 100% at the viewing angle of 0 degree, the red, green and blue light has the brightness of about 28%, 23% and 20% at the viewing angle of 60 degrees, respectively.
In this manner, the brightness of each color light is lowered, and a color coordinate value of each color light is also changed in the related art OLED display devices.
For instance, in the related art OLED display device 1, the brightness is lowered as the viewing angle increases, and the color shift occurs, thereby causing a change of the color coordinate. Therefore, there is a problem that the display quality is lowered in the related art OLED display devices according to the changes of the viewing angle